JP2657078B2 - Highly efficient mode-harmonic solid-state laser with lateral pumping - Google Patents

Description

Detailed Description of the Invention Related patent application The present application is a patent application filed on May 1, 1985
r.No. 730,002, the CIP of U.S. Pat. No. 4,653,056, issued on Mar. 24, 1987, which is a patent application Ser. No. 811,546 filed on Dec. 19, 1985; As a CIP for the current U.S. Patent No.
CI of patent application Ser.No.035,530 filed on April 7, 1987
P.

BACKGROUND OF THE INVENTION The present invention relates generally to the design of solid-state laser cavity cavities, and more particularly to the design of diode-pumped solid-state laser cavities.

Conventional optically pumped solid-state lasers use a broadband arc or flash bulb to laterally or laterally pump a solid-state laser medium in a resonant cavity. The pumping direction is transverse, that is, perpendicular to the longitudinal axis of the resonant cavity. The pumping of the medium takes place entirely such that there is not much correspondence between the pump volume and the TEMoo mode volume defined by the laser cavity. The operation is desirably in TEMoo mode. Much of the energy is provided in the region of the medium outside the volume occupied by the laser mode and does not contribute to the amplification of the laser beam. Therefore, the pumping efficiency is low (usually several%).

Laser diodes constitute an effective pumping source,
Various types of laser diodes, especially Spectra Diode La, including multiple emitters phase locked to each other
bs Model No.2410 Laser diode array such as GaAlAs laser diode array, and Sony Model Nos.S
Extended emitter laser diodes such as LD 301, 302, 303, 304 V / W are operable or have been used. U.S. Pat.Nos. 4,653,056 and 4,656,635, as well as patent application Serial No. 029,836 filed on March 24, 1987 and Serial No. 035,530 filed on April 7, 1987, have a laser diode source that extends longitudinally at the tip. A pumped solid state laser is disclosed that matches the pump volume to a desired TEMoo mode volume to optimize pumping efficiency. In a longitudinally pumped configuration at the tip, the pumping direction coincides with the longitudinal axis of the cavity and this direction can be matched to the laser mode volume. U.S. Pat. No. 4,665,529, issued May 12, 1987 and Serial No. 048,717, filed May 12, 1987, describe a method for longitudinally pumping a laser at the tip and harmonizing modes. A solid state laser diode in which a diode source is coupled to a laser rod by an optical fiber is disclosed. It is desirable to produce solid state lasers that are small, inexpensive and of high performance.

Thus, the resonator / pump structure is one of the key features for laser design and performance. Lateral pumping structures are not effective because mode coordination is not possible.
The tip pumping structure using a laser diode can achieve high efficiency because it can modulate the mode. However, the output of the conventional laser diode is usually limited to 1 W or less. Even with a higher power laser diode source, the tip pumping structure limits the amount of energy that can be used, thus limiting the laser output. This is because the output density of the pump region of the gain medium becomes too high, and the generated heat cannot be removed. So, side-pumping can deliver more energy into the medium, and mode-harmonic uses pump energy more effectively,
It would be desirable to realize a resonator structure that uses a combination of a lateral or lateral pump structure and mode matching of pump capacity to TEMoo mode volume.

Another type of laser diode is one in which a plurality of laser diode arrays are configured in a single multi-element bar structure. Laser diode array bars of this kind are typically spaced apart from each other on a 1 cm bar.
Includes 0 1W laser diode arrays. Each array has a plurality of emitters phase locked to each other. These array bars are not suitable for solid state laser tip pumping, but may be useful if the solid state laser is to be pumped laterally or laterally. However, there is little benefit to using these bars as mere substitutes for arc lamps. Therefore, the output of the laser diode array bar is set to the desired mode volume (TEMoo) in the solid-state laser material.
Therefore, it is necessary to develop a laser resonator / pump structure capable of mode matching in accordance with the requirements.

SUMMARY OF THE INVENTION For the above reasons, it is an object of the present invention to provide a solid-state laser that is laterally pumped by a laser diode and mode-tuned.

The present invention also aims to provide a mode-harmonic solid-state laser pumped by a laser diode array bar.

It is another object of the present invention to provide a solid-state laser resonator structure in which effective mode-harmonic pumping is provided by a multi-element laser diode pump source.

Another object of the present invention is to provide a laser diode pumping mode harmonic solid-state laser in which pumping is performed from the side of the gain medium.

Another object of the present invention is a solid-state laser in which a plurality of laser diode pump sources spaced apart from each other along the side surface of the gain medium are mode-tuned to the TEMoo mode.

SUMMARY OF THE INVENTION The present invention relates to a solid state laser pumped by a plurality of discontinuous laser diode pump sources located along the transverse or lateral faces of a block of laser material and mode-tuned to the mode volume of the resonator. The pump source is preferably one in which a plurality of laser diode arrays are configured in one multi-element bar structure. The effective optical pumping of this solid-state laser provides an array output in a zig-zag cavity that is bent at sharp angles in the solid-state laser material.
Achieved by mode matching to TEMoo mode. The configuration of the resonator in a zigzag shape with sharp bends between a pair of transverse surfaces or sides of the gain medium allows pump radiation to be transmitted longitudinally into the mode volume.
That is, a horizontal longitudinal pump structure is obtained. Mode matching is achieved by matching the cavity mode volume to the diffusion of the laser diode emission. Light emission of diode 1
To collimate substantially in one direction (perpendicular to the junction of the diode bar / laser gain medium), a cylindrical collimator lens, preferably a length of optical fiber, is removed from the diode bar by precision spacer means. Place them in parallel and spaced apart. The diameter of the fiber is chosen so that the pump area is consistent with the size of the laser mode. In the other direction (on the plane of the resonator), the diffusion of the laser diode emission (often with two lobe patterns) passes through the solid-state laser diode material block.
Match the bending angle of the zigzag trajectory of the TEMoo beam.

Hereinafter, the present invention will be described in more detail with reference to non-limiting specific examples based on the accompanying drawings.

Preferred Embodiments The present invention relates to a solid state laser having a resonator / pump structure that effectively couples a high average power laser diode bar to a solid state laser active medium. The solid state laser of the present invention uses a laser diode bar placed along the transverse plane or side of the laser diode material block to TEMoo.
A laser cavity having a sharp zigzag structure in the laser material block is used so that it can be matched to the modal volume. In this solid-state laser oscillator, the laser TEMo
o The overlap between the mode and the area pumped by the diode bar is optimized by any simple collimating optics and the bending angle of the cavity. The zigzag resonator cavity allows the longitudinal axis of the resonator to be substantially perpendicular to the transverse plane or sides of the laser material block rather than parallel, so that the pumping of the cavity is simply done from the tip. Not only that, it can be performed in the longitudinal direction at a plurality of intervals along the side surface.

The solid-state laser 10 shown in FIG. 1 is composed of a Nd: YAG block 12 or another solid-state laser material. Around the block 12, laser cavity forming means, such as a mirror 14 having a high laser beam reflectivity and an output coupling mirror 16 partially transmitting the laser beam, to form a laser cavity extending into the block 12.
Is arranged. The mirrors 14 and 16 are moved relative to the block 12 so that the resonant cavity structure in the block 12 becomes a zigzag 18 that is tightly bent at a predetermined bending angle between the pair of opposed side surfaces 20 and 22 of the block 12. They are arranged at an angle. In FIG. 1, the mirrors 14, 16 are both located on the same side of the block 12, but they may be located on opposite sides. Sides 20 and 22 are coated with a suitable anti-reflection (AR) coating to reflect the laser light back to block 12
Parts 20 and 22 are also provided with another coating layer that constitutes a high reflection (HR) coating. In FIG. 1, the portion of the side surface 20 between the point where the laser beam enters and the point where the laser beam exits has an HR coating, but the entire side surface 22 may be covered with the HR coating. These coatings are described in more detail below.
A diode bar 24 arranged along side 22 constitutes a pumping source. The diode bar 24 includes a plurality of separate laser diode arrays 26 spaced from one another along its length. The light emission from the laser diode array 26 of the bar 24 is selected using a fiber lens collimator 28 and the bending angle of the zigzag portion 18 of the resonator is adjusted to match the diffusion of light from the diode array 26 as described below. By doing so, it is adapted to the mode volume of the laser 10. As described below, the intra-cavity element 36 may optionally be used. In some cases, the collimator 28 may be omitted and the diode array 26 may be brought into contact with the block 12.

As a modification of the basic specific example, FIG. 2 shows a structure in which a pair of diode bars are used for pumping a solid-state laser. A separate diode bar 24 is placed along each side 20, 22 of the laser material block 12,
Pumping the zigzag portion 18 of the resonator in 12. The high reflector 14 and the output coupler 16 are located on opposite sides of the block 12 to form a resonant cavity and are oriented to provide the desired bending angle. Each diode bar includes a plurality of separate diode arrays 26, and light from these diode arrays is collimated by fiber lenses 28. The basic example described above can be modified in various other ways, for example, one diode bar, two
One, three or more may be used and placed on one or both sides of the laser material block. The cavity-forming mirrors can be located all on the same side of the block or on opposite sides. The folds in the cavity may be pumped entirely by the laser diode array, or may be pumped every other or every third and so on. Pumping of these folds can be performed on one side only or from both sides.

A great advantage of this resonator structure is that the pump volume can be very well matched to the mode volume of the resonator. The mode volume of the resonator is determined by the position and shape of the mirrors 14,16. TEMoo mode is highly desirable because it has a single lobe pattern. In this resonator structure,
A plurality of discrete pumping sources, preferably a plurality of separate diode arrays spaced apart from one another along one multi-element diode, can be arranged along one side or a pair of opposing sides. The portion of the laser gain medium that can be pumped is greatly increased, and maximum efficiency can be obtained by matching the pumping capacity of all diode arrays to the desired mode volume of the resonator. As a result, a structure with extremely high efficiency and gain is obtained.

In one preferred embodiment, the laser diode bar includes an array of ten 1W diodes located on a 1 cm piece of GaAs. Individual arrays are 200 microns wide and 1 mm
They are arranged at intervals. Such diode bars are preferred because (1) the wavelengths of these diode lasers are very well matched, since all diode lasers on the bar are fabricated on the same single crystal GaAs piece, (2) And (3) reduce the thermal load on the substrate because the diodes are arranged at 1 mm intervals, and (3) reduce the filling cost and increase the production rate because multiple diodes are combined and arranged on a single bar. On the point. However, different diode bar configurations are possible, e.g., the number or spacing of the individual arrays may vary, or a plurality of separate diode arrays may be used instead of a diode bar containing a number of separate arrays. Can be arranged along the side surface of the block. Diode arrays often have two lobe patterns with greater diffusion in a direction perpendicular to the plane of the bar. The diffusion typically has a full angle (lobe to lobe) of 7 ° on the plane of the bar, or about 0.15 numerical aperture (NA), and about 28 ° on a plane perpendicular to the bar, ie, about 28 °.
Has 0.6NA. Therefore, matching the pump beam to the modal volume in a direction perpendicular to the plane of the bar is more difficult and requires some additional optical means.

One of the preferred ways to collimate the output of the diode bar
One is to use an optical fiber as a cylindrical lens. Since a typical resonator structure has a beam 200-300 microns wide, a 300-400 micron cylindrical lens may be used. A typical multimode optical fiber has such a diameter and constitutes an inexpensive collimator as long as it can be arranged to collimate all of the spaced individual arrays. Thus, the fibers can be used to collimate a single array, but here means are needed to use the fibers to collimate the entire bar. Fig.3A ~
FIG. 3E shows a method in which the output of the laser array 26 is collimated by the optical fiber 28 and introduced into the solid-state laser block 12. Diode array 26 is spaced 1, 10, 20, 30, and 50 microns from fiber 28, which is approximately 250 microns in diameter. For said spacings of 1, 10 and 50 microns, beam collimation is not sufficient. Therefore, in order to use optical fibers for collimating the array on the diode bar, the fibers must be extended over the entire length of the bar by 30
It must be held at intervals of ± 10 microns (ie, about 20-40 microns).

As shown in FIG. 4, the optical fiber 28 substantially parallelizes the output from each laser diode 26 and
It is arranged at a distance from the diode bar 24 so as to make the pump beam 30 incident on the second side surface 22. Side 22 has a coating that is highly transparent to pump radiation (typically about 800 nm) but reflects laser radiation. The diameter of the fiber 28 results in a pump beam 30 having a diameter approximately matching (slightly smaller than) the diameter of the laser beam 32, the pump beam being oriented in one direction (perpendicular to the plane of the resonator). Select to be mode-harmonic to the mode volume. The placement of the fiber 28 with respect to the diode bar 24 is accomplished using precision spacer means 29, such as a copper stepped heat sink, focused on the diode array 26 so that the radiation is parallel. A precision spacer means 29 extends the fiber 28 over the entire length of the bar 24 to collimate the output of all arrays 26 into a single beam.
Maintain exactly parallel distance from 24. Typically, the spacing between the diode bar and the fiber should be about 20-40
Microns, the spacing between the diode bar and the YAG block is about 450 microns. Optical fibers are not perfect collimators because they have a spherical aberration that disperses the beam, but because the pump energy is absorbed into the laser medium over a relatively short distance (absorption length), the dispersion of the beam is negligible. It is. Also, the optical fiber can function as an extremely effective collimator if properly positioned in the manner described herein. Such collimated laser diode bars themselves are considered part of the present invention.

One preferred embodiment of a laser diode bar 24 that includes a fiber as an optical collimator is shown in FIG. The spacer means 29 comprises a copper or other multi-stage heat sink 38.
The diode bar 24, which includes the plurality of arrays 26, is placed on a step 40, and the fibers 28 are placed on a step 42 below (e.g., glued with epoxy). Stages 40 and 42 are intermediate stages
The exact working distance between the fiber and diode (about 30 microns), separated by 44, the full length of the bar 24 (about 1 cm)
A fiber 28 is placed against the intermediate stage to hold over. Fiber diameter is typically about 250-3
50 microns, but may have other suitable multimode fiber diameters depending on the mode volume to collimate the array. FIGS. 6 and 7 show two specific examples of the laser diode bar with a collimator of FIG. In FIG. 6, steps 42, 44 for placing the fiber 28 are formed over the entire length of the spacer 29 (heat sink 38).
In the figure, steps 42 and 44 are formed only at the end 46 of the spacer 29. Diode array 26 is formed or arranged on stage 40. The pump beam 30 from the array 26 is collimated in one direction but also has two lobe divergence in the other direction. Another way to keep the fiber at a precise distance from the diode bar is to use UV
There is a method in which the distance between the diode bar and the fiber is adjusted by using a curable epoxy, and UV is applied when parallelization becomes possible. ± 10 over bar length (1cm)
Any precision spacer or adjustment means may be used as long as the end-to-end distance can be maintained with a micron error.

As described above, if the pump beam is mode-harmonic in one direction, it must also be harmonized in the other direction (on the plane of the resonator). As shown in FIG. 8, individual diode arrays
26 are arranged on the diode bar 24 with a distance d therebetween. This distance is preferably 1 mm. Since the pump beam 30 is collimated by the fiber 28 in a direction orthogonal to the plane of the resonator, the width of the pump beam in this direction is slightly smaller than the width of the laser beam 32. On the plane of the resonator, the mode volume of the sharply bent portion of the resonator matches the diffusion of the diode array 26. Diode arrays often produce two lobe outputs, one lobe being provided to each portion of the V of the folded beam. Modal harmonization is achieved by adjusting the bending angle A. The diameter of laser beam 32 is typically about 300 microns. The geometry of the resonator and the beam mode volume are taken into account so that the beams in the bend do not overlap and therefore the laser beam exits the laser material block at the edge of the highly reflective coating on the block surface. To decide. The bending angle A is very sharp, usually 5 °. Such a structure is quite different from a conventional zigzag slab laser in which the zigzag is caused by total internal reflection (TIR) of the beam. In the present invention, the bending angle is too acute and no TIR occurs. The laser diode beam can be a single lobe, such as that caused by Sony's extended emitter laser diode. Accordingly, the distance that avoids the beam 32 from overlapping between V (ie, the two parts of V are completely separated) is equal to or greater than the absorption length of the pump beam radiation in the particular laser medium. It would be desirable to form such a zigzag structure. In this case, the bending angle depends on the mutual spacing of the diode arrays and the ability to remove the beam from the zigzag portion of the resonator. The diodes do not need to pump every bend, but may be pumped every other bend. The fold angle is particularly acute in the region of the laser beam where the pump radiation is absorbed by the gain medium, especially in the case of a single-lobe pump beam, so that the pump beam direction is best aligned with the longitudinal axis of the laser beam. Can be. The fold angle is adjusted by the cavity-forming means to provide intensity distribution of the pump source in TEMoo mode and to maximize overlap.

Various factors must be considered in the design of the cavity, as will be demonstrated in the preferred embodiment below. One is a laser material block. This block may consist of, for example, a 5 mm × 5 mm × 20 mm bar that is flattened to have a thickness of 波長 wavelength over its entire length. in this case,
The length of the fold (the distance between the two sides) is usually about 5 mm. The next factor is the laser beam mode volume. The radius of the cavity-forming mirrors is usually 100 cm, and these mirrors are placed approximately 2 cm away from the laser block. With 10 bends (20 passes in the entire block), the total cavity length is about 15 cm and the 1 / e ² beam diameter is about 300 microns. A 300 micron beam is desirable because the diode array is 200 microns and can be tuned to the mode volume for maximum pumping efficiency. However, if the cavity (mirror radius) is changed, for example, a diameter of 100
Using a 0.5cm diode bar with 10 micron 0.5W arrays (along with a narrower suitable fiber collimator) could produce a different size beam suitable for another embodiment. it can.

In order to remove the beam from the block, the beam must pass through non-reflective areas (eg, AR-coated areas) and glaze in highly reflective areas (HR-coated areas). In principle, the aperture must be approximately three times the 1 / e ² beam diameter to avoid large diffraction losses. For example, for a TEMoo beam diameter of 300 microns, the nearest edge needs to be about 500 microns. As shown in FIG. 8, beam 32 exits side surface 20 at point 48. The distance between the bends is about 1 mm. Thus, point 50 directly in front of first diode array 26 is approximately 500 microns away from exit point 48. Therefore, the HR coating must end exactly at point 50 so that the beam can exit without significant diffraction losses.

The coating is formed using a precision mask that fits the length of the bar. The mask is very thin, eg, 2 mils (0.002 inches), to sharpen the cutting edge so that the beam can exit without significant diffraction losses. The coating is a conventional optical coating. First, both sides are coated with two layers that also make up the first two layers of the AR coating, for example the HR coating. Sides without exit points can be completely covered with multiple (eg, 20) HR coatings. Then, the center part of the remaining side surfaces is covered with an HR layer using a mask.

One of the potential problems arising from the high gain is the parasitic oscillations that occur from the diode array through the laser medium block, ie during zigzag folding. One way to overcome this is to provide a highly reflective surface coating where the zigzag folds come into contact with the surface, and to create a non-resonant window in the surface portion directly in front of the diode array. It consists of coating the block surface with a series of HR stripes to give only the AR coating as much as possible. The point 52 shown in FIG. 8 (and the analogous point for each fold) is located directly in front of the array 26, so that only the AR coating is not the HR coating.
This condition can be obtained using a coating mask that forms stripes of the HR coating only where the beam reflects and does not form an HR coating at point 52 (ie, only an AR coating) (HR coating). Also, at the exit point, be slightly away from point 50). Alternatively, the entire surface may be HR coated, and at point 52 the stripes of the HR coating may be removed, for example, by a laser. Yet another alternative is to slightly wedge the normally parallel opposing sides of the same laser block. However, this method may need to compensate for the uneven spacing of the diodes.

The laser of the present invention has the advantage that it can effectively pump a solid state laser with a high power diode bar pump source by modulating the desired TEMoo mode volume. These lasers can be manufactured very small and have very high performance characteristics. The lasers of the present invention can also use any solid state laser material over a very wide range. In particular, Nd: YAG and Nd: glass are two well-known materials that are widely used in other applications. In general, the active medium must have a high slope efficiency, a wide pump bandwidth, and a high thermal conductivity. The Nd: YAG laser has very strong lines in the IR at 1.06 microns and weak lines at 0.946 and 1.3 microns. For operation with visible light, a frequency doubler can be added to the laser cavity. This frequency doubler is represented in FIG. 1 by an intracavity element 36, which generates 532 nm, 473n and 651 nm, respectively. 3 bar pump at 1.06 microns (5 at 532 nm)
By using W), an output level of 10 W can be obtained. The CW gain of the laser of the present invention is extremely high. For example, if the gain of each bending is 10 to 20%, a gain of about 7 to 8 can be obtained as a whole. Operation is CW type or Q-in cavity
It is a pulse type by adding a switch. The Q-switch is also shown in FIG. Heat dissipation may be controlled, if necessary, by placing heat sinks or other heat removal means 38 on the upper and lower surfaces of block 12, as shown in FIG. The laser of the present invention also allows the use of lower absorptivity laser materials, such as Nd: glass, due to the higher gain. This material has the advantage that the absorption line is much wider than Nd: YAG, allowing the use of a laser diode pump source without a pentier cooler. The present invention provides these extremely advantageous results in a transversely apparent (transversely longitudinal) direction.
longitudinal pumping, i.e. the pumping which is transverse to the laser block and longitudinal to the laser resonator.

It should be understood that the specific examples described above can be modified in various ways within the scope of the present invention, which is limited only by the claims.

[Brief description of the drawings]

1 is a top plan view of a mode-harmonic solid-state laser that is laterally pumped by a diode bar; FIG. 2 is a top plan view of a mode-harmonic solid-state laser that is laterally pumped by a pair of laser diode bars; 3A to 3E
The figure shows a light beam tracking graph showing the parallelized state of laser diode bars arranged at various distances from the optical fiber collimator, FIG. 4 is an end view of a laser diode having an optical fiber collimator, and FIG. FIG. 6 and FIG. 7 are end views of a laser diode bar with an optical fiber collimator.
FIG. 8 is a perspective view showing one specific example, and FIG. 8 is a top plan view of a plurality of diode arrays matched to the mode volume in a solid-state laser cavity having a zigzag structure bent at an acute angle. 12 ... laser material block, 14 ... high reflection mirror, 16 ... output coupling mirror, 18 ... zigzag part, 28 ... optical fiber.

Claims (17)

(57) [Claims] 1. A diode-pumped mode-harmonic solid-state laser comprising: a laser material block having a pair of opposed sides; and an acute angle bent at a predetermined angle between said opposed sides. Cavity forming means disposed around the block to define a laser cavity having a zig-zag resonator portion in the block;-so as to be substantially aligned with the zig-zag portion of the resonator; A plurality of laser diode pumping sources disposed proximate one of the opposing sides, wherein the bending angle is selected to substantially match the modal volume to pump radiation from the pump source. . 2. The laser of claim 1 wherein the cavity forming means includes a highly reflective mirror, a partially transmissive output coupling mirror, and a highly reflective coating formed over a portion of the opposing sides. 3. The laser of claim 1, wherein the laser diode pumping source is a laser diode bar including a plurality of separate laser diode arrays. 4. The laser according to claim 1, wherein said cylindrical collimator means is an optical fiber. 5. The laser according to claim 1, wherein the mode volume is TEMoo. 6. The laser according to claim 1, wherein the laser material is Nd: YAG. 7. The laser of claim 1 further comprising a frequency doubler located in the cavity. 8. The laser of claim 1, further comprising a Q-switch located in the cavity. 9. The laser of claim 1 further including precision spacer means for holding the fiber at a precise distance from the pump source. 10. The laser of claim 1 wherein said block has a width of about 5 mm, a mode volume diameter of about 300 microns, and a bend angle of about 5 °. 11. The laser of claim 9 wherein the pumping source is a 1 cm long laser diode bar containing ten 200 micron wide 1 W diode arrays. 12. A method for effectively pumping a solid state laser with a plurality of laser diode pumping sources, comprising: forming a block of laser material having two opposing sides; Forming in the block a laser cavity having a zig-zag resonator portion bent at an acute angle at the bending angle;-near the resonator zig-zag portion near one of the opposing side surfaces; Arranging the pumping source such that: the bending angle is selected such that the modal volume is substantially matched to the pump radiation from the pump source. 13. The method of claim 12, wherein the pump source comprises a laser diode bar including a plurality of separate laser diode arrays. 14. The method of claim 12, wherein the pump radiation is collimated by an optical fiber. 15. The method of claim 12, wherein the pump radiation is matched to the TEMoo mode volume. 16. The method of claim 12, further comprising doubling the frequency of the laser output. 17. The method according to claim 17, further comprising Q-switching of the laser.
12. The method according to 12.